The use of manikins to evaluate the thermal protection characteristics of cold water protective suits such as marine abandonment or helicopter passenger suits is not widely accepted in international standards. A full demonstration of the validity of such use requires that two questions be answered. 1. Is the heat loss from a manikin representative of the heat loss from a human under identical conditions? 2. Can the measured heat loss be related to the fall in core temperature of a human subject by appropriate models or correlations? As part of a study of the first question, the heat losses, expressed as local heat transfer coefficients or local insulation values, were compared for two thermal submersible manikins and two human subjects wearing immersion suits with three different levels of closed cell foam insulation in two floatation positions in stirred water. Both the manikins and the human subjects were instrumented with heat flow sensors, skin temperature sensors, and outer suit surface temperature sensors at 13 sites over the body. Measurements were taken over the last 30 minutes of a 60 minute exposure in a water temperature of about 12 °C and an air temperature which was in the range 15 to 19 °C. In addition to the heat flow sensor readings, the heat loss from the manikins was determined by their sectional power consumption with their skin temperatures set to a uniform 30 °C. Floatation positions were vertical with immersion to the neck, and a natural position determined by the buoyancy characteristics of the suits. This latter position was determined by measuring the height of several body points relative to the water surface on the human subjects and then arranging the manikins to the same position. The human subjects were selected to have a similar fit with the suits as the manikins. The local heat transfer coefficients showed some scatter, but no systematic variation from manikin to human or from heat flow sensor to manikin power measurement could be discerned. When the local values were used to calculate an overall resistance by the parallel method, the scatter was greatly reduced, and again no systematic variations could be discerned. The final spread in the values of overall resistance was about +/- 18%. The differences between manikin and human (+/- 12%), between humans (+/- 13%), and between manikins (+/- 6%), were all of comparable magnitude. The trend of resistance as a function of suit thickness was similar to that shown by measurements of the insulation of the suit materials on a hot plate. It is likely that the variation in local heat transfer coefficients was due to the effects of differences in fit, folds or wrinkles in the suit materials, and was essentially random. These random differences then tended to average out in the calculation of the overall resistance. It is concluded that within the scatter due to fit, folds and wrinkles, the heat loss from the manikins was a good representation of the heat loss from humans for the vertical and natural floating positions in water for suits insulated with closed cell foam.